It is known to have active implantable medical devices that are implemented as autonomous capsules implanted in a patient without any physical connection to a remote device, which remote device may be either a main implanted device (e.g., housing of a stimulation pulse generator), or a main device that is not implanted (e.g., an external device such as a programmer or a device for remote monitoring of the patient). Communication of data between the autonomous capsule and the remote (main) device is by an intracorporeal path and can be conducted by the interstitial tissues of the body, and is known as Human Body Communication (“HBC”). For this reason the autonomous capsules are often referred to as “leadless capsules” (and referred to herein more simply as “capsules”), thereby to distinguish them from those implanted electrodes or sensors that are physically placed at the distal end of a lead, the lead being traversed throughout its length by one or more galvanic conductors connecting the electrode or sensor to a generator or other device connected at the opposite proximal end of the lead.
Known leadless capsules are for example described in U.S. Patent Publication No. 2007/0088397 A1 and PCT Publication WO 2007/047681 A2 (Nanostim, Inc.) or in the U.S. Patent Publication No. 2006/0136004 A1 (EBR Systems, Inc.). These leadless capsules can notably be epicardial capsules, attached to the outer wall of the heart, or endocardial capsules, fixed to the inner wall of a ventricular or atrial cavity. Their attachment to the heart wall is usually by a projecting helical anchoring screw, axially extending from the body of the capsule and designed to screw into and penetrate the heart tissue at the implantation site.
Such a capsule typically includes detection/stimulation circuits to collect depolarization potentials of the myocardium and/or to apply pacing pulses to the site where the capsule is implanted. Such a capsule then includes an appropriate electrode, which can conveniently be an active portion of the anchoring screw. It may also incorporate one or more sensors for measuring locally the value of a parameter such as, for example, the oxygen level in the blood, the endocardial cardiac blood pressure, the acceleration of the heart wall, and the acceleration of the patient as an indicator of activity. Of course, to enable an exchange of data to a remote device, these capsules also incorporate a transmitter/receiver for wireless communication.
Several techniques have been proposed for wireless communication between a leadless capsule and a remote device, for example, to allow the remote device to receive and centralize the information collected by a capsule and in turn send the capsule appropriate instructions, as necessary. The remote device may be, for example, an implanted pacemaker, resynchronizer or defibrillator, a subcutaneous defibrillator, or a long duration data recorder. As used herein, the term “data” should be understood to mean and include digital and/or analog signals representing measured parameter values, the status of the leadless capsule and its parameterization and functionality, and any other signals, instructions and information that is to be communicated between the leadless capsule and a remote (main) device.
U.S. Patent Publication No. 2006/0136004 A1 proposes to transmit the data by acoustic waves propagating inside the body. This technique is safe and effective; it nevertheless has the drawback of requiring a relatively high transmission power given the attenuation of acoustic waves passing through the body, and allows only relatively low data rates.
U.S. Pat. No. 5,411,535 proposes a different technique, based on the use of radiofrequency waves (RF) to transmit the data. Again, a relatively high transmission power is required, and attenuation of these waves by the intracorporeal tissues is a significant barrier to their propagation.
Another communication technique was proposed by U.S. Pat. No. 4,987,897, but it is a data exchange with an external main device (e.g., programmer) performed transdermally rather than intracorporeal. This transmission is ensured for short distances between, on the one hand, the housing of a pacemaker implanted in a subcutaneous pocket, and, on the other hand, an external programmer placed near this generator. The currents therefore circulate through the skin in a zone very distant from the sensitive areas, particularly at a great distance from the myocardium, which avoids any risk of disturbance of natural or stimulated depolarization waves of the myocardium.
U.S. Patent Publication No. 2007/0088397 A1 proposes to use the stimulation pulses produced by a capsule as a vehicle for the transmission of data previously collected or created by the capsule. To this purpose, the pulse, instead of presenting a monotonic variation of voltage, is interrupted in a controlled manner for very short durations in order to create the profile of the pulse of very narrow widths whose succession corresponds to binary encoding of the information to be transmitted. This technique allows using the high energy pacing pulses to overcome the problems of attenuation in the interstitial tissue between the capsule and the device. However, it has a number of drawbacks, including:                It is limited to transmitting data by an active capsule generating pulses: in the absence of generated pulses, it is not possible to transmit any data because the generated pulse is the carrier signal or the information vehicle;        It is restricted to continuous stimulation, because otherwise the system is unable to transmit streaming data, such as electrical signals collected by the capsule or values obtained by a sensor integrated into the capsule;        It is limited to one-way communication, from the active capsule producing the pulse to the remote receiving device, but not in the opposite direction;        It has a low data rate, limited to a few bits of information per pulse, and cannot transmit information at a rate higher than that of the stimulation pulses.        
US Patent Publication No. 2002/0099423 A1 describes a wireless intracorporeal communication technique between an implanted medical device and an external device provided with electrodes in contact with the skin of the patient. The implanted device generates electrical pulse trains at a signal level that is below the cardiac stimulation threshold and delivers these pulses to electrodes to allow their propagation up to the surface of the patient's body. There, the signals are collected by the electrodes of the external device, and then decoded by the latter. This technique has several drawbacks, including a relatively high power consumption and a very high variability as a function of the load charge as seen from the implanted device between its electrodes for transmission of the pulses. Furthermore and above all, even with biphasic pulses (as described in this document), there is a high risk of residual charges, due to an imperfect balance of the positive and negative charges generated by the pulses. These residual charges produce a polarization in the tissues, thus creating a risk for the patient. For these reasons, this technique is not deemed appropriate for a permanent communication between medical devices, including communication between two implanted devices wherein the pulses would cross excitable regions of the myocardium. It is moreover proposed only for a transcutaneous communication between an implant and an external device, outside the dangerous zones. Furthermore, as it is a brief and temporary communication (the external device is, for example, used to collect from time to time the battery status of the implant), the relatively high energy consumption is not a critical factor.